1. Field of the Invention
The present invention relates generally to the field of optical imaging, and more specifically to optical systems used for microscopic imaging, inspection, metrology and lithography applications.
2. Description of the Related Art
Semiconductor wafers are composed of multiple thin film layers. These thin film layers are deposited or created and subsequently patterned one at a time. Creating and patterning each thin film layer of the wafer requires inspecting and identifying defects in the patterns for each thin film layer. The manufacturing and patterning process also entails adding a layer of photosensitive material or photoresist over the top of a thin film layer. The photoresist is then exposed to patterned light, and the photoreresist exposed to the patterned light may be removed and used as a mask. Any remaining photoresist operates as a mask to prevent etching of the underlying thin film. The underlying film may be etched and the remaining photoresist removed. The result is a pattern in the underlying thin film. Modern semiconductor reticles require advanced film products and processes, such as phase shift layers, to enhance their performance. These advanced products and processes can suffer from thin film effects.
Current wafer inspection systems primarily detect defects on individual layers using comparison techniques, such as a comparison between dies. This is commonly referred to as die-to-die subtraction. In die-to-die subtraction, the images of dies 1 and 2 are subtracted a difference may be identified between the images at location A. If the images of dies 2 and 3 are then subtracted and a difference is also found at location A, the defect is attributed to die 2 at location A.
Thin film interference effects limit the effectiveness of defect detection using the die-to-die subtraction technique, particularly in the presence of narrow band light. The thickness of the uppermost thin film layer can vary, and variations in thickness can change the reflected light level, providing a tendency to skew measurements. If a thickness variation exists from one die to the next, the thickness variation can produce additional variations in the die-to-die subtraction. Compensating for this effect requires adding the resultant difference to the defect detection threshold, thereby limiting the sensitivity of the defect detection procedure.
Interference effects can therefore adversely affect overall performance of a die to die comparison and ultimately the ability to inspect the wafer for defects. Certain illumination modes can enhance the overall inspection, and use of different illumination modes and techniques can in some cases significantly affect interference effects.
It would be beneficial to provide a system and method for use in microscopy that reduces or eliminates the effects of interference and overcomes the foregoing drawbacks present in previously known systems. Such a system may provide improved functionality over devices exhibiting those negative aspects described herein.